1. Field of the Invention
The present invention relates to a thermal printer and a thermal printing method for thermally recording an image on a recording sheet with a thermal head.
2. Background Arts
A direct thermal printer including a thermal head which thermally records an image by coloring onto a thermosensitive recording sheet provided with thermosensitive coloring layers, is known. A plurality of heating elements is aligned on the thermal head in a main scan direction. The direct thermal printer records the image on the recording sheet line by line while relatively moving the thermal head and the recording sheet in a sub scan direction. Each heating element is driven based on image data of one line to apply printing heat energy to the recording sheet.
Friction coefficient between the recording sheet and the thermal head fluctuates according to the printing heat energy. When the printing heat energy is large, the friction coefficient becomes small due to an increase of temperature of the heating elements. When the printing heat energy is small, the friction coefficient becomes large due to a decrease of temperature of the heating elements. Therefore, fluctuation of the friction coefficient becomes large at the portion where density is suddenly changed since fluctuation of the printing heat energy is large. When the friction coefficient fluctuates, transport load of the recording sheet fluctuates, thereby the transporting pitch for the recording sheet changes. There is a problem that the printing heat energy applied to each unit area changes to cause uneven density.
For example, Japanese Patent Laid-Open Publication No. 2002-67370 discloses a direct thermal printer which does not cause uneven density if the transport load fluctuation occurs due to the above change of density. In the direct thermal printer, load of the thermal head is calculated line by line, and the load fluctuation amount is calculated from the difference between the load of the line to be recorded and that of the adjacent line. And the printing heat energy for the line to be recorded is corrected on the basis of the load fluctuation amount. Therefore, even when the transporting pitch for the recording sheet changes due to the transport load fluctuation, the printing heat energy is corrected according to the change thereof, and the uneven density is prevented.
When the printing heat energy is corrected based on the transport load fluctuation, it is necessary to precisely study a relationship between dynamic friction coefficient of each heating element to the recording sheet and the printing heat energy, and a pressure distribution of each heating element to the recording sheet. Thereby, a problem arises in that it is necessary to obtain various data by actually operating each printer, thereby time and labor is required for obtaining data. Moreover, there is a problem that it is impossible to correct the printing heat energy sufficiently even when the printing heat energy is corrected by using the obtained data, since the transport load fluctuation is suppressed by correction of the printing heat energy, not by directly acting on the recording sheet.